In order to integrate an entire transceiver system of a communication system into a single integrated circuit (IC), attempts have been made to replace conventional gallium arsenide (GaAs) power amplifiers with high-efficiency complementary metal-oxide semiconductor (CMOS) power amplifiers. However, radio frequency (RF) characteristics, e.g., non-linear characteristics, of CMOS power amplifiers generally impede their use in communication systems. Although efficiency of CMOS power amplifiers approach that of GaAs power amplifiers, linearity of CMOS power amplifiers has not been significantly improved, particularly due to intrinsic non-linearity of CMOS field-effect transistors (FETs) used in CMOS power amplifiers.
Non-linear distortion of a transistor device may be categorized as amplitude modulation-amplitude modulation distortion (“AM-AM distortion”) and amplitude modulation-phase modulation distortion (“AM-PM distortion”). In general, when amplitude of a voltage swing becomes close to knee voltage of the transistor device, transmission conductance determining gain of the transistor device decreases, and therefore the gain decreases. A CMOS FET has a higher knee voltage than a GaAs heterojunction bipolar transistor (GaAs HBT) or a GaAs FET, for example. Therefore, reduction in the gain of a CMOS FET generally occurs relative to a lower output power than reduction in the gain of a GaAs-based amplifier, and therefore a CMOS power amplifier tends to have a narrower linear operation region than a GaAs power amplifier.
In order to provide desired linearity, a CMOS power amplifier must be operated in a large back-off power region (or voltage region) that is narrower than an operation region of a conventional power amplifier, e.g., a GaAs power amplifier. However, when the CMOS FET of the CMOS power amplifier is operated in a back-off power region, efficiency of the CMOS power amplifier generally decreases. Therefore, it may be necessary to reduce non-linearity of the CMOS power amplifier in order to increase the efficiency of the CMOS power amplifier.